Thin-film, crystalline silicon on glass (CSG) technology has attracted a great deal of interest as an active material in photovoltaic devices and other large area applications such as thin film transistors (TFT). When applied to solar cells, CSG technology can deliver very low manufacturing costs, however a key element in achieving useful efficiencies which result in a low $/W manufacturing cost is effective, low-cost hydrogenation for defect passivation.
In order to obtain technologically useful materials and devices, the deleterious electrical activity of defects must be reduced. Hydrogenation, the incorporation of atomic hydrogen, has been shown to be an effective method for passivating defects in semiconductors such as silicon. Numerous defects are susceptible to hydrogen passivation, including Si dangling bonds and strained Si—Si bonds at grain boundaries, dislocations, point defects with deep levels such as Au and Fe, and oxygen and its complexes. Commonly used hydrogenation methods include directed ion beam or Kaufman sources, and plasma sources in either the direct or remote configuration. It has long been known that H-induced damage can be of critical importance, motivating the use of remote plasma sources. In the PV industry, the most widely used hydrogenation technique uses a hydrogen-rich SiNx layer deposited by PECVD to simultaneously provide an antireflection coating, surface passivation and following H diffusion into the silicon during the contact firing step, bulk passivation. The efficiency of hydrogenation strongly depends on the passivation technique, passivation temperature, and the type and density of defects in the target material—with low-cost CSG presenting a significant challenge.
CSG modules are fabricated by depositing amorphous silicon onto textured glass after which the silicon is crystallised, for example by solid-phase crystallisation, or laser crystallisation. The resulting highly defected poly-Si is then improved by rapid thermal annealing and hydrogenation. Various methods of hydrogenation have been tried, however many existing methods are slow and the speed with which the hydrogenation step can be completed has a significant effect on the cost of production of a CSG module.
A problem with hydrogenation methods where the target must be immersed in a plasma is that regardless of the speed of the process itself loading the target into the plasma chamber, bringing the target and chamber up to temperature before the process can begin, and reversing these steps before unloading adds a significant time overhead to the process.
A further issue that arises with hydrogenation is that the ‘in-diffusion’ of atomic hydrogen that occurs during the hydrogenation process will be counteracted by an “out-diffusion” in some circumstances. In particular, if the target is removed from the plasma whilst still at a high temperature, out-diffusion may result in all of the previously in-diffused hydrogen being lost.
Another factor which must be considered is the occurrence of dopant (e.g. boron) deactivation and surface damage which will occur when a silicon film is exposed to atomic hydrogen, especially at lower temperatures. Such damage can include etching and creation of surface defects (e.g. platelets and voids) and has an adverse affect on device performance and can cancel the benefits of hydrogenation. Surface damage may become a significant problem if a target remains exposed to atomic hydrogen at temperatures below approximately 300-350° C., (depending on the intensity of atomic hydrogen and the CSG material quality) for significant periods of time.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.